Step 1: You can begin with downloading the Android 5.1 Lollipop AOSP Beta ROM for your device on the PC.

Step 2: Now, download the Gapps files.

Step 3: With the help of a USB cord, connect your device with PC. You can transfer the above downloaded files to the internal storage of your device now. Ensure that you’ve transferred it to the rooted folder.

Step 4: Remove your device from the PC. Go to the Fastboot folder along with boot.img and system.img files. You must have Android ADB tools on your PC.

Step 5: In the download mode, turn off your device. Press and hold Volume button together for doing this.

Step 6: You can go further with the flashing of boot.img and system.img files available in the fastboot folder.